The present invention relates to an improved process for making a heat resistant nickel-base polycrystalline superalloy forged part.
The invention more particularly relates to a process for making forged component parts of a turbo machine, such as a forged single-piece gas turbine wheel.
Nickel-base superalloys are widely used for producing gas turbine parts.
The nickel-base superalloys are usually used for two different types of application in the gas turbine technology.
A first type of application concerns generally bulky parts such as turbine wheels which must exhibit good mechanical properties in the temperature range from ambiant to 700.degree. C.
In this first type of applications, the part is generally produced by forgeing from remelt ingots and working or from densified alloy powders. The resulting parts usually have a fine grain structure. The main drawbacks of this route is the high sensitivity of the resulting part to intergranular rupture. This intergranular rupture occurs principally around 700.degree. C. due to oxidation at the grain boundaries and phase precipitations at these grain boundaries.
A second type of application concerns relatively thin turbine parts such as turbine blades which must be operable at very high temperatures, typically ranging from 900 to 1200.degree. C.
These second type of parts are usually produced by casting and heat treating alloys having a very high content of refractory metals. The resulting parts have a coarse grain structure and do not exhibit the required mechanical properties in use at the lower temperature of 700.degree. C. or less.
Prior art nickel-base or nickel-chrome superalloy forged parts have been found not to have the required creep properties when used at temperatures higher than 650 to 700.degree. C.
Forged parts have been produced from nickel-base polycrystalline superalloys and in particular have been sold under the trademark UDIMET 720.
One of these alloys has the following composition in percentage by weight:
chromium: 18% PA1 cobalt: 14.7% PA1 molybdenum: 3% PA1 tungsten: 1.25% PA1 titanium: 5% PA1 aluminium: 2.5% PA1 zirconium: 0.03% PA1 carbon: 0.035% PA1 boron: 0.033% PA1 a) providing a nickel-base superalloy having the following composition: PA1 and eventual impurities, wherein the weight ratio B/C is equal to or greater than 1.1; PA1 b) forging said nickel-base superalloy to obtain an as-forged part; PA1 c) submitting the as-forged part of step b) to a solution heat treatment at a temperature ranging from 10 to 30.degree. C. above the .gamma.' phase solvus temperature; PA1 d) quenching the solution heat treated forged part resulting from step c) at a quenching rate above 100.degree. C./minute; PA1 e) submitting the part resulting from step d) to a first heat aging at a temperature ranging from 650.degree. C. to 750.degree. C. for at least 16 hours; and PA1 f) submitting the part resulting from step e) to a second heat aging at a temperature of 800 to 850.degree. C. for at least 4 hours.
Forged parts are typically made from this type of superalloy by remelting under a vacuum and/or under a slag, forging and heat treating.
Different heat treatments are classically used depending on the subsequent applications, but of which at least one includes heating beyond 1100.degree. C. for a period of 2 to 4 hours.
There occurs at these temperatures a precipitation of the secondary phases and in particular of the carbides at the grain boundaries which has an adverse effect on the good mechanical behavior of the forged parts when used in a temperature range between 700 and 900.degree. C., in particular by a deterioration in the area of the grain boundaries.
A first attempt to overcome this problem consisted in trying to modify the geometry of the grain boundaries to obtain the so-called "zig-zag" boundaries by acting on the precipitation of the phases. This technique is however harmful to the performances in the temperature range of between 700 and 800.degree. C.
A second manner of proceeding consists in modifying the grain boundaries so as to trap the harmful carbides at low temperatures by the addition of boron.